1. Field
Embodiments of the present invention relate to a lithographic apparatus, a stage system and a stage control method.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithography, use is made of control systems to control a position, velocity, acceleration and/or other position parameters of a movable part, such as a substrate stage (also referred to as substrate table or wafer table) or a support (also referred to as mask table or mask stage). In such control systems, use is made of a feedback control loop to provide an accurate response of the control system. In addition, to achieve a fast response, use may be made of a feedforward, such as acceleration feedforward. Such acceleration feedforward may provide a desired force to move the stage or other movable parts in question, the acceleration feedforward being determined by multiplying the desired acceleration with a mass of the stage or other movable parts. However, the stage may not be infinitely stiff, i.e. is not a rigid body, and internal resonance modes, such as flex modes, may react to the feedforward, thereby possibly resulting in a deformation of the stage during acceleration or deceleration.
In the feedback loop, a position measurement is provided to determine a position of the stage. The position measurement however does not take place at a same location point of the stage as the location point where the actuator force is applied, which as a consequence results in a position error when the above deformation of the stage occurs. To be able to cancel such error, an additional force may be applied, using a snap-feedforward, which (almost) instantaneously provides for a movement of the stage. The snap-feedforward may in a control diagram representation be provided as an additional feedforward path parallel to the acceleration feedforward, and being inputted with a corresponding setpoint signal. In this document, the term snap is to be understood as a double time derivative of the acceleration. By application of the snap-feedforward, the point at the stage where the position is measured thereby more precisely corresponds to the desired position setpoint. The deformation has however not been cancelled by this snap-feedforward. In the deceleration phase, an opposite effect may take place, whereby the deformation may be directed in an opposite direction.
Thus, despite application of the snap-feedforward, deformation of the stage may take place in dynamic situations, and the instantaneous movements of the stage by application of the snap-feedforward may add high frequency components to the movement of the stage, and may create a residual error when the snap-feedforward is applied.